1. Field of the Invention
The present invention generally relates to an optical module cage mounting structure, and particularly relates to a structure for mounting an optical module cage on a printed circuit board of a plug-in unit to be attached to a device casing.
2. Description of the Related Art
FIG. 1 schematically illustrates a communication device 1. The communication device 1 comprises a device casing 2 in which plural plug-in units 10 are aligned and mounted. An optical fiber 5 is connectable from the outside to the plug-in units 10.
There is a strong demand for devices such as the communication device 1 to initially include only circuits for necessary lines for minimizing initial investment, to allow adding optical modules without suspending services each time lines are added, and to allow replacing existing optical modules with other optical modules for different transmission distances.
In each plug-in unit 10, plural optical module cages 12-1, 12-2, and 12-3 are aligned and mounted at the front edge of a printed circuit board 11 such that optical modules 20 (FIGS. 2A - 2E) are attachable thereto. Connectors 13-1, 13-2, and 13-3 are mounted on the printed circuit board 11 and accommodated in the rear parts of the optical module cages 12-1, 12-2, and 12-3, respectively. While, for purposes of simplicity of explanation, the three optical module cages 12-1-12-3 are shown in FIG. 1, more than three optical module cages are actually provided.
Each of the optical module cages 12-1-12-3 is formed in a box shape with an insertion slot 12-1a at an end, and has a lock piece 12-1c with a lock hole 12-1b in the position facing the insertion slot 12-1a. 
In FIG. 1, the optical module 20 is inserted in and attached to only the uppermost optical module cage 12-1. A plug 6 provided at the end of the optical fiber 5 is connected to the optical module 20. The other optical module cages 12-2 and 12-3 are empty. In case of adding lines, additional optical modules 20 are inserted in and attached to the optical module cages 12-2 and 12-3. This configuration enables reducing initial investment and adding optical modules 20 when adding lines.
FIGS. 2A-2E show the optical module 20. The optical module 20 is a SFP (Small Form-Factor Pluggable) type and comprises a pair of optical plug sockets 21 and 22 at an end, a light emitting element, a light receiving element, and a photoelectric transducer (not shown) thereinside, and a card edge terminal group section 23 at the other end. A lock release knob 24 slidable in a direction A (FIG. 2D) and a lock projection 25 are disposed on the bottom face of the optical module 20 at the side where the optical plug sockets 21 and 22 are provided.
The optical module 20 is inserted, from the end where the card edge terminal group section 23 is provided, all the way into the optical module cage 12-1 such that the card edge terminal group section 23 is connected to the connector 13-1. The lock projection 25 is engaged by the lock hole 12-1b, so that the optical module 20 is attached inside the optical module cage 12-1.
When the lock release knob 24 is pushed and slid in the direction A, the lock piece 12-1c is flexed so as to disengage the lock projection 25 from the lock hole 12-1b. The unlocked optical module 20 is pulled out and removed from the optical module cage 12-1.
As described above, the communication device 1 is configured to allow adding optical modules 20 when adding lines, and to allow replacing the optical modules 20 with other modules when replacing interfaces.
Currently, there is a demand for increasing capacity of backbone networks to handle growing data traffic due to an increase of data and popularization of the Internet. Accordingly, electronic devices for optical communication systems that multiplex optical signals are required to have a higher density, a greater data transmission capacity, and more sophisticated functions. To meet such demands, as many optical modules as possible need to be mounted occupying less space so as to increase the number of lines per device.
FIG. 3A shows a related-art optical module cage 30. The optical module cage 30 comprises a cage body 31 in which the optical module 20 is inserted and leads 32 extending from the cage body 31.
The leads 32 are inserted into through holes in a printed circuit board 11 and soldered to the printed circuit board 11 such that, as shown in FIG. 3B, the optical module cage 30 is mounted on the printed circuit board 11 with its bottom plate 33 in contact with a face 11a of the printed circuit board 11.
<Patent Document 1>Japanese Patent Laid-Open Publication No. 2005-116751
This configuration has the following four problems.    (1) As shown in FIG. 4, optical module peripheral electronic components 40 of filter circuits, etc., for the optical modules 20 are mounted outside the area on the printed circuit board 11 where the optical module cages 30 are mounted. Accordingly, spaces 41 for mounting the electronic components 40 need to be provided between the adjacent optical module cages 30, which makes it difficult to mount a sufficient number of optical module cages 30.    (2) For mounting more optical module cages 30, optical module cages 30-1 and 30-2 may be mounted, as shown in FIGS. 5A and 5B, in an opposing relationship on opposite faces of the printed circuit board 11. However, if the thickness of the printed circuit board 11 is 3 mm or less (as in many cases), a lock release knob 24-1 of an optical module 20-1 attached to the optical module cage 30-1 and a lock release knob 24-2 of an optical module 20-2 attached to the opposing optical module cage 30-2 interfere with each other and make it difficult to perform unlocking operations for removal of the optical modules 20-1 and 20-2.    (3) The optical modules 20 include therein components that are easily affected by heat. As these components need to be effectively cooled during use, forced air cooling is used for the optical module cages 30. However, in the case where plural optical module cages 30 are aligned and mounted as shown in FIGS. 6A and 6B, air passes through only at the upper side of the optical module cages 30 without sufficiently cooling the optical module cages 30 disposed downwind, especially. Therefore, the optical modules 20 attached to the optical module cages 30 disposed downwind are often subjected to severe thermal conditions.    (4) The optical module cage 30 is usually mounted, as shown in FIG. 3B, with its insertion slot side projecting outward beyond the edge of the printed circuit board 11 so as to prevent a lock piece 30c from interfering with the printed circuit board 11. If the entire part of the optical module cage 30 is located on the printed circuit board 11, the area of the printed circuit board 11 facing the lock piece 30c needs to be cut off to prevent interference with the lock piece 30c. Cutting off the area of the printed circuit board 11 facing the lock piece 30c reduces the area for mounting the optical module peripheral electronic components 40.